Because of the paucity of clinical and scientific information, there are no criteria for determining when the diseased, traumatized or operated thoracic spine is clinically unstable. Accurate assessment would avoid the risks of unnecessary surgery as well as the unhappy neurological catastrophe that may occur with inadequate treatment. We have been successful in developing useful criteria for the recognition of clinical instability in the cervical spine. The objective of this proposal is to use the same basic technique with modifications to develop similar criteria for the thoracic spine. A second objective is to accumulate quantitative data about the stiffness properties of the thoracic spine before and after various anatomic structures have been destroyed. Motion segments (two adjacent vertebrae with their soft tissue) are fixed and physiological loads (50% body weight) are applied as to simulate either flexion or extension. With the specimens in a high humidity chamber, motion segments are tested by cutting the ligamentous structures and the facet articulations from posterior to anterior in some specimens and vice versa in others, with some loaded in flexion and some in extension. Through the use of displacement gauges with x-ray monitoring, the motion in translation and rotation is recorded before and after each ligament is cut up to failure of the motion segment. Because several loads are applied (12.5, 25, 37.5, and 50% body weight), it is possible to determine the changes in stiffness of the motion segments. This experiment will provide objective quantitative criteria for the assessment of clinical instability in the thoracic spine. It should give information about: (a) the ligaments which when cut render the thoracic spine unstable, (b) the number of mm. of translation or the number of degrees of rotation that represent instability, (c) also the changes in the stiffness properties of the thoracic spine will be quantitated. This information will be of considerable value to the patient by improving physician management decisions about the diseased, traumatized or post surgical thoracic spine.